Self-copying sheets comprise at least two superimposed sheets whose contacting faces are coated with a layer each. Usually, the top sheet comprises on its back surface a layer of microcapsules which contain electron-donating or nucleophilic colorless or pale colored leuco dyestuffs, and a bottom sheet with a coated upper surface layer containing electron accepting or electrophilic developers. Multicopying stacks are formed by alternating these layers, i.e. the aforementioned bottom sheet has on its back surface another nucleophilic layer, and so on. The said microcapsules are locally destroyed under the action of a pressure, typically a letter of a typewriter or the stylo of a pen, and a color forming reaction takes place between the nucleophilic leuco dyestuff and the electrophilic acceptor on the upper side of the second sheet. A reproduction of the writing on the upper sheet is thus obtained on the upper surface of the lower sheet.
In some cases, there may be area portions on the second sheet where no reproduction, and thus no color formation, should appear on the second sheet or copy. Since there is no means to avoid the destruction of the microcapsules under pressure, the said areas should be neutralized on the surface of the second sheet. This is generally accomplished by a locally confined printing with a nucleophilic desensitizing ink.
The most appropriate printing methods for the application of such desensitizing inks to the copying paper are the dry offset, the wet offset or lithographic, the flexographic, and the typographic methods.
The dry offset method uses an elastomer plate having elevated active printing surfaces which are inked. The ink on the inked surface portions is transferred to a rubber cloth which will then deposit the ink on the sheet to be printed. This method requires only a low pressure for the transfer of the ink to the upper surface of a sheet having a back microcapsule coating (CFB). Thus, there is only little risk to destroy said microcapsules.
The wet offset or lithographic printing method employs a metal plate where the areas to be printed are oleophilic, and the areas not to be printed have a hydrophilic character. The oleophilic areas are inked from an ink trough followed by an inking train, whereas humidifying rollers wet the hydrophilic areas from a wetting water trough. The ink on the oleophilic areas of the plate is then transferred to a rubber cloth and further deposited on the surface to be printed.
A balance between the ink feed and the water feed must be reached on the plate in addition to the possible emulsion balance of the water within the ink. These balances are responsible for the printing sharpness, and they are in physicochemical relation with the hydrophilic-lipophilic balance (HLB).
The flexographic method uses relief surfaces formed by elastic cliches of rubber or resilient synthetic materials fixed to a printing roller. In a similar manner, the typographic method employs a matrix having elevated surface portions. The elevated portions of the two printing systems will be inked and then transmit the ink on the substrate to be printed.
The printing methods mentioned above are well known to the one skilled in the art until the least detail, and they will thus not be repeated here.
The known desensitizing inks, also named neutralizing inks, are designed to be printed by offset on certain predetermined surface areas of the accepting sheet of a chemical copying set where the sheet or the sheets already comprise at least one electrophilic accepting layer on the upper surface. It will be necessary that the desensitizing ink has a nucleophilic character to be able to neutralize the electrophilic effect of the accepting composition.
Numerous organic compounds have already been suggested and used as active nucleophilic components of desensitizing inks which are capable of neutralizing the electrophilic components of the accepting layer on predetermined surface areas in order to prevent a color formation under the effect of a pressure which destroys the microcapsules containing the nucleophilic leuco dyestuff. These nucleophilic active components are selected according to the particular printing method and the composition of the printing ink. The choice is limited since the active compound must fulfill a plurality of conditions well known to the man of the art.
Generally, the nucleophilic property of the active component is based on the presence of free electron pairs attached to electronegative atoms, especially oxygen and nitrogen, capable of combining with a pair of electron holes of an electropositive atom such as carbon, sulfur, boron, etc. Polyalkylene glycols, glycerol, long-chain fatty quaternary ammonium salts and long-chain amines have already been proposed as active nucleophilic components of desensitizing inks. These desensitizing compounds, however, have a number of inconvenients. For example, the desensitizing effect may decline under the influence of heat, humidity or light. Other compounds have a tendency to migrate on those areas of the accepting layer where a coloration should occur. Most known compounds undergo a browning after a certain time. The amines and their simple derivatives have generally a strong and disagreeable smell; some of them are toxic or allergenic.
It has been tried to overcome these drawbacks in proposing other active nucleophilic compounds. For example, the published European patent application no. 0 088 466 discloses a nucleophilic active compound which is an alkoxylated derivative of an organic compound having an active hydrogen atom such as alcohol, phenol, fatty acid, or amine, polyalkoxylated with ethylene oxide and/or propylene oxide. The German Patent DE-C3-25 26 592 describes polyalkoxylated amines which are at least partially esterified or etherified on the terminal hydroxyl groups stemming from the alkoxylation of mono and polyamines.
However, although the neutralizing compounds of the German patent DE-C3-25 26 592 bring about an improvement as to the odor and to the hydrophobicity of these compounds, it has been found that the neutralizing or desensitizing power of these compounds is comparable or even weaker than that of known compounds or to compounds which are sold as components of desensitizing inks. This fact renders the deposit of a high amount of ink necessary to obtain a perfect desensitizing effect.
However, the use of desensitizing inks has disadvantages by itself. Firstly, the drying of an ink is the longer the more the printing is thick, and the drying time is an exponential function and not a linear function of the thickness. This fact is of the highest importance since, if the drying speed is too slow, an off-set of the printed sheets which are deposited in stacks at the end of the printing machine is observed, or the off-set of successive layers when the paper is wound on rolls.
Secondly, and from an economical viewpoint, the total cost of a desensitizing printing depends before all on the price of the ink, and desensitizing inks are very expensive.